The present invention relates to a process for the surface treatment of a titanium dioxide pigment, to a novel titanium dioxide pigment and to the use of this novel pigment and of those obtained by the abovementioned process, in paper manufacture.
It is known that titanium dioxide, in particular in its rutile form, may be used advantageously as an opacifying pigment in paper manufacture. This is because titanium dioxide is a white pigment which has a high refractive index; it is among the compounds which, for an optimum particle size (generally 0.2 to 0.3 .mu.m), best diffuse light.
The incorporation of titanium dioxide into paper usually consists in mixing the titanium dioxide with cellulose fibres which are predispersed in water. More or less efficient binding by electrostatic attraction between the cellulose fibre and the titanium dioxide particles can then take place. The cellulose fibre is by nature negatively charged.
The opacity of the paper pigmented with titanium dioxide depends in particular on the titanium dioxide content of the paper; it is thus a function of the level of binding to the cellulose fibres and the physicochemical retention of the titanium dioxide.
The titanium dioxides currently used as opacifying pigments have a physicochemical retention which may appear insufficient. The physicochemical retention quantifies the ability of the titanium dioxide to be retained on the cellulose fibres of the paper; the level of retention is defined, in this application, as the ratio of the amount of titanium dioxide effectively bound to the cellulose fibres to the total amount of titanium dioxide used during the incorporation. This low level of physicochemical retention is economically damaging to the process, poses problems of pollution and of effluent recycling, and decreases the final opacity properties of the paper or of the laminate.
Moreover, the capacity for physicochemical retention of the titanium dioxide pigments changes as a function of the hardness of the water used to manufacture the paper. Thus, a pigment having a satisfactory level of physicochemical retention for a soft water will not necessarily have this same property for a hard water.